The data set collected in 2018 is divided into two subsets, S1 and S2,
which correspond to the periods before (20\(\%\)  of the data set) and
after (80\(\%\)  of the data set) the installation of the new final
collimator COL. The subset S2 is further divided into six categories
corresponding to equal 5 GeV\(/c\)  bins of pion momentum, \(p_{\pi ^+}\) ,
in the range 15–45 GeV\(/c\) . The subset S1 is considered as a separate
category and is integrated over \(p_{\pi ^+}\)  due to its small size. A
dedicated selection is applied to each category, which improves signal
sensitivity. Data sets from 2016–2017, analyzed in [1]}
and [2]}, are added as two separate categories, each
integrated over \(p_{\pi ^+}\) , for a total of nine categories.
